Buildings are commonly equipped with thermal insulation in order to provide comfortable living conditions inside the building. Thermal insulation has added advantages in reducing the consumption of energy and reducing carbon dioxide emissions that occurs as a result of heating and cooling buildings. These advantages have led to the installation of increasing amounts of insulation in construction of new buildings and during renovation of old buildings. Because of considerations such as appearance, cost, ease of installation and protection of the insulation, insulation is often installed in walls, ceilings and/or roofs between the interior finished surface and the exterior sheathing of the building in cavities between the framework and the interior and exterior surfaces.
In addition to insulation, buildings are also constructed with various air barrier materials that are installed between the sheathing and the exterior cladding to limit air and water infiltration into the structure of the building. These air barriers minimize heat loss through draft flow.
In addition to air barriers, vapor barriers are also used for controlling flow of moisture. For example, the moisture flow is generally from inside to outside when the exterior air is cold and dry (winter) and from outside to inside when the air is hot and humid (summer). In regions where these conditions prevail, it is common practice to use a vapor barrier such as polyethylene (PE) film on the warm side of the insulation, generally behind the gypsum wallboard, to prevent moisture diffusion from the interior heated space to the exterior cold environment. Such vapor barriers reduce the potential for moisture condensation in the interior wall cavity space as the water vapor is exposed to the cold exterior temperatures. Moisture condensation causes wood decay, promotes mold growth, reduces insulation value, and consequently must be eliminated whenever practical. Although the use of PE film as a vapor barrier reduces moisture condensation during exterior conditions of cold temperatures and low humidity, the same vapor barrier increases the likelihood of moisture condensation at the insulation-PE film interface during hot and humid exterior conditions where moisture flow is from outside to inside. Therefore, it is desirable to have a material that acts as a vapor barrier during winter but facilitates vapor permeation during hot, humid summer conditions. It is also desirable to have a vapor barrier that can become vapor permeable under humid conditions in the wall cavity caused by unexpected uncontrolled events like water leaks into the wall cavity, to facilitate drying of the wall cavity by inward moisture flow as well as outward flow.
U.S. Pat. Nos. 6,808,772; 6,878,455 and 6,890,666 disclose applications of a polyamide building liner material that has a water vapor diffusion resistance (WVDS) of from 2 to 5 meters diffusion-equivalent air space width at a relative humidity (RH) of an atmosphere surrounding the vapor barrier between 30% and 50% and a WVDS of less than 1 meter diffusion-equivalent air space width at a relative humidity between 60% and 80%. See also, U.S. Patent Application Publication 2003/0215609 (a moisture-adaptive vapor-barrier film comprising PE and acrylic ester with a WVDS of from 0.5 to 100 meters diffusion-equivalent air space width at an RH of an atmosphere surrounding the vapor barrier between 60% and 80%) and WO2002/070351 (use of ionomers for sealing insulating materials having a WVDS of from 1 to 20 meters diffusion-equivalent air space width at a RH of an atmosphere surrounding the vapor barrier of 25% and a WVDS of from 0.02 to 0.7 meter diffusion-equivalent air space width at a relative humidity of 72.5%).